This invention relates in general to vehicle brake systems, and in particular to vented reservoirs for master cylinders.
Vehicle braking systems generally include a driver operated brake pedal which is connected to a master cylinder. The master cylinder is in fluid communication with brakes at associated wheels of the vehicle. Operation of the brake pedal actuates the master cylinder to supply pressurized brake fluid to the brakes, thereby decelerating the vehicle or preventing the vehicle from moving from a stopped position. A booster may also be provided between the brake pedal and the master cylinder to assist the pedal effort by the driver of the vehicle.
Master cylinders generally include a body having a cylindrical bore formed therein. In a typical tandem (two piston) master cylinder, a primary piston and a secondary piston are slidably disposed within the cylindrical bore. The pistons are operatively connected to the brake pedal by an input rod extending through one end of the master cylinder. The body of the tandem master cylinder and the pair of pistons define primary and secondary fluid chambers. The primary and secondary chambers are in fluid communication with separate primary and secondary brake hydraulic circuits, respectively. The primary and secondary brake hydraulic circuits are connected to certain ones of the brakes on the vehicle.
To operate the brake system, the driver depresses the brake pedal which inputs a force to the input rod. The force from the input rod acts against the primary piston. The force is selectively applied to urge the primary piston to pressurize the brake fluid in the primary chamber. The primary chamber is in fluid communication with the primary brake hydraulic circuit. The secondary piston forms one wall of the primary pressure chamber, and is thus acted upon by the pressure in the primary pressure chamber. The secondary piston is thus urged by the pressure in the primary pressure chamber to pressurize the brake fluid in a secondary pressure chamber. The secondary chamber is in fluid communication with the secondary brake hydraulic circuit.
Master cylinders are generally provided with a reservoir for storage of brake fluid. The brake fluid within the reservoir is selectively in fluid communication with the primary and secondary chambers of the mastery cylinders. When the master cylinder is not actuated, the reservoir is in fluid communication with the primary and secondary chambers. When the master cylinder is actuated, such as by depression of the brake pedal, the communication between the reservoir and the primary and secondary chambers is closed to allow the chambers to pressurize. The reservoir functions to store a volume of brake fluid to compensate for the volumetric changes in the primary and second chambers and the hydraulic brakes lines of the hydraulic circuits. The volumetric changes can be caused by the wear of the brake linings of the wheel brakes, thermal expansion and contraction of various components of the brake systems, and leakage.
The reservoirs can be vented or non-vented. In vented reservoirs, the fluid stored in the reservoir is vented to atmospheric pressure so that as the fluid level of the brake fluid within the reservoir changes, a pressure differential between the interior of the reservoir and the atmosphere does not occur. A pressure differential between the interior of the reservoir and the atmosphere may interfere with the proper function of a brake system requiring a vented reservoir.
Commonly, venting is provided by the use of one or more relatively small openings formed through a reservoir cap. The cap generally closes off the opening of a relatively large filling port of the reservoir. Due to movement of the vehicle and external forces acting on the fluid within the reservoir, fluid can escape through the relatively small opening in the cap. It has been known to provide a series of internal buffing plates formed in the cap, to provide a baffled or labyrinth-like passageway through the cap to help prevent the escape of fluid. The cap can also include a sealing gasket disposed in the vent passageway, wherein the sealing gasket has one or more slits formed therein which open when a relatively low pressure differential occurs between the reservoir and the atmosphere. However, because of space limitations and packaging restraints of the location of the master cylinder and reservoir within an engine compartment of the vehicle, the slotted sealing gaskets and labyrinth passageways may not suitably prevent the escape of fluid during all vehicle orientation and operating conditions. It is also desirable to prevent water and other external contaminants from entering the reservoir through the cap openings. The slotted sealing gaskets and labyrinth passageways may not suitably prevent the entrance of these contaminants.